The present invention relates generally to apparatus and methods for providing repeatable measurements of volume within an enclosed chamber. More specifically, the present invention provides methods and apparatus for calibrating volume measurement in a plethysmographic measurement system.
The assessment of body composition, including measurement of fat and fat-free mass, provides physicians with important information regarding physical status. Excess body fat has been associated with a variety of disease processes, such as cardiovascular disease, diabetes, hypertension, hyperlipidemia, kidney disease, and musculoskeletal disorders. Low levels of fat free mass have been found to be critically adverse to the health of certain at-risk populations, such as the elderly, infants, and those suffering from muscle wasting diseases.
Assessment of body composition has also been found to be useful in the context of evaluating and improving athletic performance. Generally, athletes require a high strength to weight ratio to achieve optimal athletic performance. Because body fat adds weight without a commensurate increase in strength, low body fat percentages have been emphasized within many athletic fields. However, too little body fat can result in deterioration of both health and athletic performance. Thus, accurate measurement of body composition has been found extremely useful in analysis of athletic performance.
A variety of methods are currently used in the assessment of body composition. One common method is a skinfold measurement, typically performed using calipers that compress the skin at certain points on the body. While non-invasive, this method suffers from poor accuracy on account of variations in fat patterning, misapplication of population specific prediction equations, improper site identification for compressing the skin, poor fold grasping, and the necessity for significant technician training to administer the test properly.
Another method employed is bioelectric impedance analysis (xe2x80x9cBIAxe2x80x9d). Bioelectric impedance measurements rely on the fact that the body contains intracellular and extracellular fluids that are capable of conducting electricity. By passing a high frequency electric current through the body, BIA determines body composition based on the bodies"" measured impedance in passing current, and the known impedance values for human tissue. However, the accuracy of this method is greatly affected by the state of hydration of the subject, and variations in temperature of both the subject and the surrounding environment.
The most common method currently used when precision body mass measurements are required is hydrostatic weighing. This method is based upon the application of Archimedes principle, and requires weighing of the subject on land, repeated weighing of the subject under water, and an estimation of air present in the lungs of the subject using gas dilution techniques. However, hydrodensitometry is time consuming, typically unpleasant for the subjects, requires significant subject participation such as repeated, complete exhalation of air from the subject""s lungs, requires considerable technician training and, due to the necessary facilities for implementation, is unsuitable for clinical practice. Further, its application to populations who would particularly benefit from body-mass measurement, such as the obese, elderly, infants, or cardiac patents, is precluded by the above concerns.
One technique offering particular promise in performing body mass measurement is the use of plethysmography. Plethysmographic methods determine body composition through application of Boyle""s law to the differentiation in volume between the volume of an empty measurement chamber, and the volume of the chamber with the subject to be measured inside. Examples of this technique are disclosed in U.S. Pat. No. 4,369,652 issued to Gundlach, U.S. Pat. No. 5,450,750 issued to Abler, U.S. Pat. No. 4,184,371 issued to Brachet, and U.S. Pat. No. 5,105,825 issued to Dempster. This procedure, in contrast to hydrodensitometry, generally does not cause anxiety or discomfort in the subject, and due to the ease and non-invasiveness of the technique, can readily be applied to populations for whom hydrodensitometry is impractical.
However, such plethysmographic systems require very exact volume measurements to yield valid body composition results. In particular, calibration of the measurement chamber equipment used to generate the volume measurements for body composition analysis is necessary for achieving accuracy, on account of very small differences in measured volume yielding large differences in computed body composition. Although some efforts have been made in the field of calibration for plethysmographic systems, these methods are typically complicated, inexact, and/or inconvenient for the medical technicians who conduct plethysmographic body composition measurements by requiring manual activation and implementation of the calibration.
For example, Dempster, U.S. Pat. No. 5,108,825, discloses the use of a calibration volume structure that is manually placed in a plethysmographic reference chamber. However, this process is slow, cumbersome, and requires active participation by the medical technician to calibrate the system.
Ganshorn, U.S. Pat. No. 5,626,005, discloses a method of calibration for a plethysmographic chamber for measuring the volume of a subject""s thorax-lung system. The method disclosed by Ganshorn involves the use of a harmonically oscillating piston pump that generates pressure fluctuations that simulates a test subject""s breathing, and relies on these pressure fluctuations to calibrate a chamber pressure gauge based on the simulated breathing. However, this method is unnecessarily complex and not generally applicable to calibration of plethysmographic chambers used in the measurement of body composition.
Therefore, it would be desirable to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that provides accurate calibration of the measurement system.
It would further be desirable to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that does not require active, manual participation of medical technician to conduct the calibration.
It is an object of the present invention to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that provides accurate calibration of the measurement system.
It is another object of the present invention to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that does not require active, manual participation of medical technician to conduct the calibration.
These and other objects of the present invention are accomplished by proving computer assisted methods and apparatus for calibration of a plethysmographic measurement system using a calibration volume chamber.
The present invention generally consists of a calibration volume chamber of known, fixed volume coupled to a plethysmographic measurement chamber in a plethysmographic measurement system, wherein a computer system is used to calibrate the measurement system prior to conducting a volume measurement of a test subject, by measuring the chamber volume before and after opening (or alternatively, before and after closing) an electronically controlled valve that connects the controlled calibration volume to the plethysmographic chamber, and comparing the measured chamber volumes based on the known reference volume.
In one embodiment of the present invention, the actuation assembly for opening and closing the valve in response to a signal from the computer system is a cam and motor assembly coupled to a shaft that is mounted to the valve by means of a ball joint.
In a second embodiment of the present invention, the actuation assembly for opening and closing the valve in response to a signal from the computer system is a solenoid coupled to a shaft that is mounted to the valve by means of a ball joint.